Two standards for the broadcast and transmission of analog video signals are the National Television System Committee (NTSC) standard used in the United States, and the Phase Alternation by Line (PAL) standard used in many European countries. Under these standards, video images are displayed on a television screen using a video signal to control an electron beam that is scanned across the television screen. When the bottom of the screen is reached, the electron beam moves back to the top of the screen. During this transition, no video data may be transmitted. The portion of a video signal where no video information is broadcast is called the Vertical Blanking Interval (VBI) of the video signal.
In the NTSC format, 525 lines of video are displayed in each frame, with colors transmitted on a sub-carrier frequency of 3.579545 MHz. The NTSC standard defines a frame rate of 30 frames per second, half of the frequency of the 60 Hz electric power used in the United States. Each frame includes two interlaced fields of video data. The fields are displayed at the rate of 60 per second, and each field includes 262.5 lines of video image data. With each pass of the electron beam from the top of the display to the bottom, every other line of the display is updated. Thus, two passes are used to display a complete frame.
FIG. 1 shows the pattern that an electron beam follows according to the NTSC standard. The image begins with display of the first field of a frame at point 101. The electron beam follows the indicated path across the display to the end of row 3, then the beam retraces across the third raster line to the left side of the display. Next, the electron beam travels down to row 5 and then retraces across the fifth raster line to the left side of the display. This process continues until the electron beam reaches the bottom of the screen at point 102. The electron beam then travels to point 103 and begins the process again, this time tracing the even raster lines instead of the odd raster lines. When the beam reaches the bottom of the screen at point 104, the entire frame is complete; the beam retraces to point 101 and begins displaying the next frame.
Under the NTSC standard, the VBI includes the time required for the electron beam to reach point 101 from point 104 and the time required to reach point 103 from point 102. The video signal is ordinarily blanked out during this interval, preventing distortion of the image displayed. Because video is blanked, the video signal can be used to transmit data during this interval, perhaps including non-video information. Under the NTSC standard, the VBI yields 21 lines of data that are not displayed on the screen; however, lines 1-9 are reserved for vertical synchronization and line 21 is reserved for closed-captioning. Thus, 11 lines (lines 10-20) are available for transmitting data that may or may not relate to the displayed image.
The PAL standard uses a format similar to NTSC; however, PAL is used in Europe where the electrical power system is 50 Hz instead of 60 Hz. Consistent with this frequency, the frame rate for PAL is 25 frames per second, each field includes 625 lines of video, with and a sub-carrier frequency of 4.43 MHz is used to broadcast color information. The PAL standard reserves lines 6-22 in the first field of a frame and lines 319-335 in the second field of a frame to transmit VBI data, thus yielding 16 lines of data per frame for VBI information.
One commercial application of VBI data transmission is an electronic programming guide (EPG).
Another application that is being developed provides for the transmission of Internet Protocol packets via the VBI of an analog video signal. This proposal has been published as RFC 2728, an Internet Engineering Task Force (IETF) standards document.